# Switching capacitors in a guitar effects pedal

I've built a linear power booster - schematic here: http://pedalparts.co.uk/docs/LPB-V3.pdf

I did it on veroboard:

You'll note that it gives you the option of having different capacitor values to turn the pedal from a 'clean' boost to a treble booster. Ideally I would like to make one with a 3 way switch - ON-ON-ON (double pole) to be able to switch between capacitors.

Now I just tried this with a 3 way slide switch that looked like this:

Forgive the crude diagram, but it was a 2 pole switch so I used the other pole to do a similar thing with the other cap (C2, not actually electrolytic, just ceramic)

When I tried it, the first position with the high value (100n) cap sounded ok, but the others were choked and thin sounding. It's a guess, but are the other two (unused) capacitors acting like decoupling caps and filtering out loads of the frequencies? The sound is more trebley, but it's very quiet. Is there a better switching mechanism I could use so that I don't have to have 3 capacitors connected to the same place? Maybe a diode or six?

• We prefer to see real schematic diagrams, rather than pictorials. You should have a ground/common somwhere, where the battery negative terminal and the bottom terminal of the pot are connected. – Peter Bennett Oct 5 '16 at 23:07
• What were the values of the other caps? – user207421 Oct 6 '16 at 0:39
• ${f_c} = {1 \over {2\pi RC}}$ – Ignacio Vazquez-Abrams Oct 6 '16 at 0:53
• @PeterBennett, Row I is ground, I just omit ground wires in my pictures. The circuit functions so the grounding is not the issue. – TCassa Oct 6 '16 at 10:02
• @EJP, the default is 100nF (for both capacitors in the circuit) and the alternates are 10nF and 2.2nF. – TCassa Oct 6 '16 at 10:02

First a schematic diagram rather than pictorial.

This is a very basic single amplifier stage with a voltage gain of approx R3/R4.

Its more of a bass cut than a treble boost (you're cutting out the bass notes) so you lose the bass below the cut off frequency allowing more of the treble to come through.

C3 and R1//R2 form a high pass filter (CR). With C3 = 100nF the sound is OK.

When you switch in the 10nF your are raising the lower cut off by a factor of 10 and the 2n2 by a factor of 45 (ish).

Let's assume that the 100nF gives a lower cut off of 500Hz. Then 10nF gives a cut off of 5kHz and the 2n2 gives about 22.5kHz (i.e. above the threshold of human hearing). Little wonder you can hardly hear anything.

A further complication is the value of C2. If this is too low it also acts as a high pass filter.

Some possible solutions:

(1) Make sure C2 large enough to give a full frequency range output and not acting as a second high pass filter.

(2) Choose values for C3 in a much closer range (2:1). Try a slightly larger value than 100nF as well (say 220nF (more bass), 100nF (clean) and 47nF (trebely))

• Don't forget that in this specific amplifier design, there is a very low valued $R_E$ that affects the $R_1\vert\vert R_2$, significantly (even with a high beta BJT.) Also, little-re will be significant because $V_E$ of $Q_1$ is close to ground -- I estimate somewhere between $100\:\textrm{mV}$ and $140\:\textrm{mV}$. So the gain will be responsive to temperature. (Several similar designs I've seen use a bypass cap on $R_E$ [high gain depends on $\beta$ and even more responsive to temps] and use the pot to pick up the differences.) – jonk Oct 6 '16 at 16:57
• Many thanks Jim - this is very helpful. But I notice the schematic in the link recommends switching BOTH capacitors, whereas you think I should leave the last one at something high. What would a sensible C2 be? Perhaps 220nF? – TCassa Oct 6 '16 at 17:40
• @TCassa 220nF would be more than enough for C2 giving a break frequency (high pass) around 50Hz, you could go to 100nF. – JIm Dearden Oct 7 '16 at 9:27